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explain NATURAL JOIN in SQL

Of course. Let's break down NATURAL JOIN in SQL, including what it is, how it works, and why it's generally a bad idea to use in practice.

What is a NATURAL JOIN?

A NATURAL JOIN is a type of SQL join that automatically connects two tables based on all columns that share the same name. You don't specify the join condition (the ON clause); the database figures it out for you by finding matching column names.

In essence, it's a shortcut for an INNER JOIN where the join condition is an equality check on all identically named columns.

Key Characteristics:

Implicit Join Condition: You don't write ON tableA.column = tableB.column. The database does this for you implicitly for every column that has the same name in both tables.
Inner Join Behavior: It only returns rows where the values in the common columns are equal.
No Duplicate Columns: The common columns appear only once in the final result set.

A Simple Example

Let's imagine you have two tables, Students and Courses.

Students Table:

student_id	student_name	course_id
1	Alice	101
2	Bob	102
3	Charlie	101
4	Diana	104

Courses Table:

course_id	course_name	professor
101	Intro to SQL	Dr. Codd
102	Data Structures	Dr. Wirth
103	Algorithms	Dr. Cormen

Both tables have a column named course_id. A NATURAL JOIN will automatically use this column to link them.

The Query:

SELECT student_name, course_name, professor
FROM Students
NATURAL JOIN Courses;

The Result:

The database finds the common column (course_id) and effectively runs the query ... ON Students.course_id = Courses.course_id.

student_name	course_name	professor
Alice	Intro to SQL	Dr. Codd
Charlie	Intro to SQL	Dr. Codd
Bob	Data Structures	Dr. Wirth

Alice and Charlie were matched to "Intro to SQL" because their course_id is 101.
Bob was matched to "Data Structures" because his course_id is 102.
Diana is not in the result because her course_id (104) does not exist in the Courses table.
The "Algorithms" course is not in the result because no student is enrolled in it (course_id 103).

The Dangers: Why You Should Avoid `NATURAL JOIN`

While it seems convenient, NATURAL JOIN is considered dangerous and is strongly discouraged in professional software development for two main reasons:

1. It Hides the Logic and Reduces Readability

When you look at a NATURAL JOIN, you have no immediate idea how the tables are being joined. You have to stop and manually inspect the schema of both tables to find all the columns with matching names. This makes the query difficult to understand and maintain.

An explicit INNER JOIN ... ON is always clearer:

-- This is crystal clear. No ambiguity.
SELECT s.student_name, c.course_name, c.professor
FROM Students s
INNER JOIN Courses c ON s.course_id = c.course_id;

2. It is Extremely Brittle and Can Lead to Unexpected Bugs

This is the most critical danger. The behavior of your query can change unexpectedly if someone alters the table schema.

Let's continue our example. A few months later, a developer adds an audit column named last_updated to both tables.

New Students Table:

student_id	student_name	course_id	last_updated
1	Alice	101	2023-10-27
2	Bob	102	2023-10-26

New Courses Table:

course_id	course_name	professor	last_updated
101	Intro to SQL	Dr. Codd	2023-09-01
102	Data Structures	Dr. Wirth	2023-09-01

Now, if you run the same NATURAL JOIN query again:

SELECT student_name, course_name, professor
FROM Students
NATURAL JOIN Courses;

The database now sees two common columns: course_id and last_updated. The implicit join condition becomes:

ON Students.course_id = Courses.course_id AND Students.last_updated = Courses.last_updated

Because the last_updated dates will never match, your query will now return zero rows! This can silently break your application in a way that is very difficult to debug. You intended to join only on course_id, but the database changed the logic without warning.

The Better Alternatives

Always be explicit about your join conditions.

INNER JOIN ... ON (Best Practice): This is the clearest, safest, and most maintainable way to write a join. It is completely unaffected by the addition of new columns.
```
SELECT s.student_name, c.course_name
FROM Students s
INNER JOIN Courses c ON s.course_id = c.course_id;
```
INNER JOIN ... USING (A Safer Alternative): If you want the convenience of not repeating the column name and having it appear only once in the result, USING is a much safer option than NATURAL JOIN. You explicitly list which column(s) to join on.
```
-- This still joins on course_id, even if other matching columns exist.
SELECT student_name, course_name
FROM Students
INNER JOIN Courses USING (course_id);
```

Summary

Feature	`NATURAL JOIN`	`JOIN ... ON`	`JOIN ... USING`
Clarity	Very Poor	Excellent	Good
Safety	Very Dangerous & Brittle	Very Safe	Safe
Join Keys	Implicit (all common names)	Explicit (you define the logic)	Explicit (you name the columns)
Recommendation	Avoid at all costs in production code.	The universally recommended standard.	A good, safe shorthand if columns have the same name.

explain USING clause in the statement above

Of course. Let's focus specifically on the USING clause from the previous SQL explanation.

What is the `USING` Clause?

The USING clause is a shorthand way to specify the columns for an equi-join (a join based on equality) when the columns you are joining on have the exact same name in both tables.

Think of it as the perfect middle ground between the dangerous, implicit NATURAL JOIN and the verbose, explicit JOIN ... ON.

Syntax:

SELECT ...
FROM table1
JOIN table2 USING (common_column1, common_column2, ...);

Let's Revisit the Example

Here are our tables again:

Students Table:

student_id	student_name	course_id
1	Alice	101
2	Bob	102

Courses Table:

course_id	course_name	professor
101	Intro to SQL	Dr. Codd
102	Data Structures	Dr. Wirth

The `JOIN ... USING` Query:

SELECT student_name, course_name, course_id
FROM Students
INNER JOIN Courses USING (course_id);

How It Works:

Explicit Instruction: You are explicitly telling the database: "Join the Students and Courses tables by matching the values in the column named course_id."
Implicit Condition: The database translates this into the equivalent ON clause: ON Students.course_id = Courses.course_id.
Clean Output: A key feature is that the common column (course_id) appears only once in the result set. You don't get two separate course_id columns.

Result of the USING query:

student_name	course_name	course_id
Alice	Intro to SQL	101
Bob	Data Structures	102

Notice there's just one course_id column. If you had used JOIN ... ON, you would typically have to specify which table's course_id you want (e.g., SELECT ..., s.course_id) to avoid ambiguity.

`USING` vs. `ON` vs. `NATURAL JOIN`

This table makes the differences crystal clear:

Feature	`JOIN ... ON`	`JOIN ... USING`	`NATURAL JOIN`
How it works	You write the full join condition explicitly.	You name the common columns to join on.	Implicitly joins on ALL columns with the same name.
Column Names	Can be different (`ON a.id = b.a_id`).	Must be the same in both tables.	Must be the same in both tables.
Clarity	Excellent. The logic is 100% visible.	Good. Clearly states the join keys.	Poor. Hides the logic.
Safety	Very Safe. Unaffected by schema changes.	Safe. Unaffected by unrelated schema changes.	Dangerous. Query behavior can change silently.
Join Columns in Output	Duplicated (e.g., `Students.course_id` and `Courses.course_id`).	Appears only once.	Appears only once.

The Key Advantage of `USING`

The main advantage of USING is that it gives you the safety and clarity of being explicit while providing the convenience of not having to type out the full ON condition and dealing with a duplicated join column in the output.

Let's go back to the "dangerous" scenario where we added a last_updated column to both tables.

NATURAL JOIN would fail by trying to join on course_id AND last_updated.
JOIN ... USING (course_id) would continue to work correctly because you explicitly told it to only use course_id for the join. It would ignore the new last_updated column.

Example with Multiple Columns

The USING clause can also handle composite keys (joins on multiple columns), as long as they all share the same names.

Imagine an OrderItems table and a Shipments table.

OrderItems Table: order_id, product_id, quantity

Shipments Table: order_id, product_id, shipment_date

To join them, you would use:

SELECT *
FROM OrderItems
JOIN Shipments USING (order_id, product_id);

This is equivalent to the more verbose:

SELECT oi.order_id, oi.product_id, oi.quantity, s.shipment_date
FROM OrderItems oi
JOIN Shipments s ON oi.order_id = s.order_id AND oi.product_id = s.product_id;

The USING version is cleaner and easier to read.

Conclusion

Use USING when: The columns you are joining on have the same name in both tables, and you want a clean, safe, and less verbose query.
Use ON when: The column names are different, or you need more complex join logic (e.g., > or BETWEEN). ON is the most flexible and universally applicable standard.
Avoid NATURAL JOIN in production code.